1. Field of the Invention
The present invention relates to a method of estimating the fuel/air ratio for each cylinder of an internal-combustion engine, in particular an injection engine.
2. Description of the Prior Art
Knowledge of the fuel/air ratio, characterized by the mass of fuel to the mass of air, is important for all vehicles, whether equipped with gasoline or diesel engines. In the case of a gasoline engine, the fuel/air ratio influences the nature of the emissions. A slightly rich mixture creates an increase in the CO and HC emissions, whereas a slightly lean mixture increases the NOx emissions. It is therefore of great importance to precisely control the fuel/air mixture cylinder by cylinder in this type of engine running with a total engine fuel/air mixture (average of the 4 cylinders) around stoichiometry so as to limit the emissions. In the case of conventional diesel engines or of gasoline engines running under stratified combustion conditions, combustion generally takes place with a lean mixture (fuel/air ratio below 1) and it is less sensitive to a precise fuel/air ratio adjustment. Depollution by deNox catalysis of this type of engine however requires temporary maintenance, for some seconds, of a slightly rich mixture in order to bleed the NOx trap, prior to coming back to normal running conditions with a lean mixture. Depollution by deNox catalysis therefore requires precise control of the fuel/air ratio cylinder by cylinder in order to guarantee the required fuel/air level during this stage. Finally, engines running according to new combustion types, in particular HCCI diesel engines, among which the NADI™ concept developed by the assignee can be rated, work with very high recycled burnt gas ratios and therefore confined fuel/air ratios, which also make them very sensitive to a precise adjustment of the fuel/air ratio of each cylinder.
In order to control more precisely, and in particular individually, injection of the fuel masses into the cylinders, reconstruction of the fuel/air ratio in each cylinder is necessary. Since installing fuel/air ratio probes at the outlet of each cylinder of a vehicle cannot be done considering their cost price, setting an estimator working from measurements provided by a single proportional probe placed in the common part of the exhaust advantageously allows separately knowing the fuel/air ratio of each cylinder. An engine control can thus, from the reconstructed fuel/air ratios, adjust the fuel masses injected into each cylinder so that the fuel/air ratios are balanced in all the cylinders.
In the description hereafter, the invention is illustrated by the example of a supercharged diesel engine equipped with a NOx trap, where the probe can be placed at the turbine outlet and upstream from the NOx trap. The measurement provided by this probe is used for total engine control of the mass injected into the cylinders during the rich phases, each cylinder receiving then the same mass of fuel. The present invention however applies to all engine types having one or more proportional probes downstream from the junction of several cylinders.
French Patent 2,834,314 describes a model achieved, then observed and filtered by means of the Kalman filter. This model contains no physical description of the mixture in the manifold and does not take into account of the highly pulsating flow rate phenomena.
Estimation of the fuel/air ratio is only conditioned by the coefficients of a matrix, coefficients which have to be identified off-line by means of an optimization algorithm. Furthermore, a different adjustment of the matrix, therefore an identification of its parameters, corresponds to each working point (engine speed/load). This estimator thus requires substantial acquisition testing apparatus (with 5 fuel/air ratio probes) and is not robust in case of engine change.